Carburization is a process through which carbon is added to the surface of metal articles. It is carried out by exposing the metal article to a carbon rich environment and maintaining the article at a temperature that allows for diffusion to transfer carbon atoms into the metal which is typically steel. This temperature will be sufficient to maintain the steel as austenite that has a face-centered cubic structure and which has a high solubility for carbon. The carburization will typically be carried out until the carbon content of the steel has been increased to the desired level which will typically be between about 0.5% and 1%. Hardening of the high-carbon surface layer of the steel article is then accomplished by quenching the article to form martensite having enhanced hardness and wear resistance.
It is often desirable to carburize the surface of articles to improve hardness and wear resistance without compromising the strength or toughness of the underlying steel in the article. For instance, it is normally desirable to carburize the outer surface of gears to improve wear resistance while maintaining the strength of the steel in the body of the gear. In such a scenario, a medium-carbon or low-carbon steel can be used in making a gear which is carburized to increase the carbon content of only its surface.
Historically, carburization was initially performed by simply packing the steel part in a carbon powder in a suitable container and maintaining it at an elevated temperature for a time period which was sufficient to allow the carbon to diffuse into the steel article. This technique, known as pack carburization, was later improved by packing the steel part in charcoal granules that were treated with an activating chemical, such as barium carbonate (BaBO3). In this improved pack carburizing process, the activating chemical promotes the formation of carbon dioxide which in turn reacts with excess carbon in the charcoal to produce carbon monoxide. The carbon monoxide then reacts with the low-carbon steel surface of the article to form carbon atoms that diffuse into the steel. This increases the carbon content of the steel near the outer surface of the metal article, but does not, as is the case with all carburization procedures, increase the hardness of the steel. Accordingly, the metal article is subsequently quenched to attain the desired level of hardness. Pack carburization is an extremely effective method of increasing the carbon content near the surface of metal parts. However, pack carburization is a very slow, time consuming process. Attaining uniform and consistent results is another problem frequently encountered when utilizing pack carburization.
Over the years, a number of improved techniques for carburizing steel articles have been developed. These techniques include gas carburizing, plasma carburizing, salt bath carburizing and liquid carburizing. Gas carburization involves heating the steel article being treated to a temperature above about 1,550° F. (843° C.) to form austenite and maintaining the article at that temperature in a carburizing gas atmosphere for a time that is sufficient to increase the carbon content near the surface of the article to the desired level. The time required for the carbon to diffuse into the steel will typically vary from about four hours to about ten hours. The carburizing gas atmosphere will typically be a mixture of hydrogen and methane in an inert gas or a mixture of carbon monoxide and carbon dioxide in an inert gas. In the first case, the hydrogen/methane ratio and in the second case the carbon monoxide/carbon dioxide ratio is adjusted to give the desired carbon concentration on the surface of the steel being treated. Uniform results can be attained by carefully controlling the ratio of reactive gases and the carburization temperature. Gas carburizing leads to a uniform result with carbon being diffused consistently over the surface of the metal part being treated. However, gas carburization is a time consuming and expensive procedure.
Vacuum carburization utilized a single-component atmosphere consisting solely of a simple hydrocarbon in a gaseous state, such as methane. Vacuum carburization is carried out at low pressure under an oxygen-free environment and offers the advantage of being able to utilize higher carburization temperatures without the risk of surface or grain-boundary oxidation. This, in turn, leads to higher levels of carbon solubility in the austenite formed and to increased rates of carbon diffusion. The time required to attain the desired carbon level at the surface of the part is accordingly reduced.
Even though vacuum carburization eliminates some of the complexities of gas carburization, it is not universally applicable to the treatment of all metal parts. This is because the rate of flow of the carburizing gas into deep recesses in the part is quickly depleted at the low gas pressures used. This leads to an insufficient level of carbon penetration in the steel at such recesses in the structure of the part. Thus, the treated part will not have the desired level of hardness and wear resistance at such points on the surface of the part. It should be noted that this problem typically cannot be overcome by simply increasing the pressure of the carburizing gas because sooting usually results in such a scenario.
In plasma carburization, carbon ions are given a positive charge and the steel part being carburized is provided with a negative charge and acts as the cathode to which the positively charged carbon ions are drawn. Plasma carburization rapidly introduces carbon into the surface of the part and also provides fast diffusion kinetics. Plasma carburization also offers the advantage of providing a very uniform carburization of the part even in cases where the part has deep recesses or other surface irregularities which are difficult to carburize using gas or vacuum carburization techniques. However, plasma carburization is expensive and requires sophisticated equipment.
Salt bath carburization involves immersing the steel part in a molten carbon rich salt bath. Such salt baths traditionally use cyanide as a major component of the carbon rich bath. Safety concerns have limited salt bath carburization procedures from coming into widespread utilization. Additionally, salt bath carburization requires relatively expensive specialized equipment.
In liquid carburization, a liquid hydrocarbon is employed as the source of carburizing gas. The liquid hydrocarbon can be an aliphatic or an aromatic hydrocarbon such as hexane, cyclohexane, benzene, or toluene. Oxygenated hydrocarbons such as alcohols, glycols, and ketones are also commonly used as the liquid hydrocarbon source. In such liquid hydrocarbon carburization procedures, the liquid hydrocarbon is fed into a carburization furnace containing the steel part or parts being carburized and volatilizes almost instantaneously at the temperature of the furnace. The vapors of the liquid hydrocarbon source dissociate thermally to provide a carburizing atmosphere that typically contains carbon monoxide, carbon dioxide, methane and other lower alkanes. In the case of oxygenated hydrocarbons water vapor is also typically produced. The flow of the liquid hydrocarbon source into the furnace is adjusted to accurately attain the desired level of carburization. However, liquid hydrocarbon carburization is a slow process which makes it capital intensive and expensive.
There is a need for a carburization technique that leads to uniform results and which can be carried out quickly on a cost effective basis. However, no conventional carburization procedure offers all of these benefits.